Waveguide apparatus for illumination systems

ABSTRACT

A waveguide such as optical fiber is to receive a primary light in a longitudinal (propagation) direction. The fiber has formed therein scattering structures that re-direct the propagating primary light out of the waveguide, for instance in a transverse direction. A photo-luminescent layer absorbs the re-directed primary light to thereby emit secondary wavelength converted light having a different wavelength than and broader bandwidth than the primary light, resulting in white illumination light, being the secondary light combined with any unabsorbed primary light. Other embodiments are also described and claimed.

RELATED MATTERS

This application is a continuation of U.S. application Ser. No.14/113,905, filed on Dec. 3, 2012, which is a National Stage Entry ofPCT/IB2012/000617, filed on Mar. 28, 2012, which claims the benefit ofthe earlier filing date of U.S. Provisional Application No. 61/480,216,filed Apr. 28, 2011, entitled “Waveguide Apparatus for IlluminationSystems.”

FIELD OF THE INVENTION

An embodiment of the invention generally relates to a cylindricalwaveguide apparatus that as part of a distributed illumination systemmay render white illumination in a more directional, uniform intensityand more energy efficient manner. Another embodiment is a method formanufacturing the apparatus. Other embodiments are also described.

BACKGROUND

In a basic fiber illumination system, a light source injects light intoan optical fiber and the fiber then serves to transport the injectedlight to a remote, desired location. At the remote location, the fiberis exposed, typically at its end surface, so that the light can escapeand illuminate the remote region that is outside and near the endsurface of the fiber. More recently, a fiber-based illumination systemhas been suggested that has multiple regions each with a different indexof refraction, and this can be used to allegedly deflect the propagatinglight out a side of the fiber along its length. See U.S. Pat. No.5,905,837 to Wang, et al. A 360 degree or omnidirectional deflectionpattern has also been suggested, through the use of combined changes inthe ratio of index of refraction between the core and the cladding, andabsorption and scattering coefficients in the fiber. See U.S. Pat. No.6,714,711 to Lieberman, et al. A subsequent effort has suggested the useof a blazed diffraction grating in the core of the fiber, fordiffracting the light out of the fiber, and a convex lens structureoutside of the fiber is to receive the diffracted light and establish alinear illumination field. See U.S. Pat. No. 7,437,035 to Carver, et al.However, the efficacy of such techniques in efficiently producingillumination light having broadband visible content (also referred tohere as white light) while ensuring its uniformity along the length ofthe fiber is not apparent.

SUMMARY

An embodiment of the invention is a light waveguide apparatus that maybe part of a distributed illumination system that produces illuminationhaving broadband visible content (white light) while allowing easycontrol of the color temperature and intensity uniformity of theillumination along the length of the waveguide. The apparatus has awaveguide that is to transport or guide primary light to a remotelocation, i.e. remote from a source of the primary light. The waveguidecontains a number of scattering structures, which serve to re-distributeor redirect the propagating primary light out of a side surface of thewaveguide and with a desired radiation pattern. The radiation patternmay be directional, for instance having at least one predetermined lobeof radiation having a radial spread of less than 360 degrees and at adesired radial position, and it may be positioned as desired in thelongitudinal direction. A medium or layer of photo-luminescent materialis provided, preferably outside of and running longitudinally along thewaveguide, to absorb the re-directed primary light, and as a result emita secondary, wavelength-converted light having a different wavelengththan the primary light. The primary light should be quasi-singlewavelength or monochromatic, as produced by for example a laser or asingle-color light emitting diode (LED) that is tuned to a primaryabsorption band of the photo-luminescent medium. This produces broadbandillumination light (also referred to here as white light) due to thesecondary wavelength converted light being combined with any unabsorbedre-directed primary light, in a direction that, in one embodiment, maybe transverse to the propagation axis of the waveguide, at the remotelocation. The illumination light may, in essence, be a combination ofthe wavelength converted secondary light (which may be of broaderbandwidth than the primary light) plus any unabsorbed portion of there-directed primary light. The waveguide apparatus also enablespositioning the primary light source away from the light conversionlocation, so that thermal dissipation at that location may be reduced.Other embodiments are also described.

The waveguide be any suitable optical fiber such as single clad,multi-clad, and photonic-crystal or micro-structured fiber, which mayyield better illumination efficiency due to lower parasitic reflectionsand lower manufacturing costs. In one embodiment, the optical fiber mayhave a core layer and a cladding layer. The scattering structures (e.g.,micro-diffusers or reflectors) are preferably laser-induced structurespreviously formed inside the fiber, either entirely in the core layer,or partially in the core and partially in the cladding. These scatteringstructures are designed to redirect the primary light in accordance witha desired radiation pattern that cuts through the outer or front sidesurface of the fiber; the radiation pattern may thus have a shapedefined in part by certain characteristics of the scattering structures.The photo-luminescent layer may be shaped to be concentric with thefiber, and/or may be shaped to receive, partially or completely (e.g.,oriented perpendicular to) the radiation pattern of the redirectedprimary light. In particular, light conversion efficiency may beenhanced by (a) matching the geometry of the waveguide and that of thephoto-luminescent layer (e.g., conforming an incident surface of thephotoluminescence layer to the radiating or outer surface of thewaveguide), and (b) adapting the refractive index difference between amaterial of the waveguide and a material of the photo-luminescentmedium, particularly where the waveguide material has a refractive indexthat is about equal to or lower than that of the photo-luminescentmedium.

An optional reflector may be positioned behind the waveguide and may bedesigned to reflect some of the radiation pattern of the re-directedprimary light, together with any incoherent secondary light. Forinstance, the reflector may have a curved reflecting surface, and may besized and positioned to be concentric with the cylindrical waveguide. Ingeneral however, the reflector may have a larger radius than that of thecylindrical waveguide, or it may have a non-circular shape, e.g.,rectangular, V-shaped. In addition, or perhaps as alternative, there maybe a reflector positioned between the photo-luminescent medium and thewaveguide, that is designed to reflect the secondary light, and let passthe primary light. This may be part of an intermediate medium or layerthat is formed between the outer side surface of the waveguide and theinner face of the photo-luminescent layer.

In one embodiment, homogeneous illumination, i.e. relatively uniform orfixed intensity and/or color quality, may be obtained along the lengthof the waveguide, by processing the laser-induced scattering structuresso as to locate them close to each other, e.g. from a few nanometers toa few microns, and to vary their scattering strength as a function oftheir position along the length of the waveguide. This may enable thescattering strength to compensate for the inevitable power loss sufferedby the primary light as it propagates along a scattering region or zonein the waveguide. Also, by using multiple primary light sources atdifferent wavelengths, wherein the light from each of these lightsources is scattered out by a respective scattering structure and thenabsorbed by a respective section of photo-luminescent layer, animprovement in the color quality of the illumination light may bepossible. In general, the location, shape, size, strength, orientationand periodicity of the scattering structures, both along the primarylight propagation axis as well as across or transverse to it, may beselected or adapted to yield a desired characteristic for theillumination light, e.g. an intentionally non-homogeneous illuminationpattern.

The above summary does not include an exhaustive list of all aspects ofthe present invention. It is contemplated that the invention includesall systems and methods that can be practiced from all suitablecombinations of the various aspects summarized above, as well as thosedisclosed in the Detailed Description below and particularly pointed outin the claims filed with the application. Such combinations haveparticular advantages not specifically recited in the above summary.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the invention are illustrated by way of example andnot by way of limitation in the figures of the accompanying drawings inwhich like references indicate similar elements. It should be noted thatreferences to “an” or “one” embodiment of the invention in thisdisclosure are not necessarily to the same embodiment, and they mean atleast one.

FIGS. 1a-1b are sectional side views of a waveguide apparatus inaccordance with an embodiment of the invention, showing various types ofscattering structures of various shapes and periodicity.

FIG. 1c is a graph of an example spectrum of the illumination lightprovided by the waveguide apparatus.

FIGS. 2a-2c are sectional views of example radiation patterns for there-directed primary light.

FIG. 3a is a sectional end view of an example cylindrical waveguideapparatus in a fully integrated version.

FIG. 3b is a sectional end view of the example waveguide apparatus ofFIG. 3a superimposed with references to the behavior of the primary andsecondary light.

FIG. 4 is a perspective view of an example cylindrical waveguideapparatus for being coupled to multiple, different wavelength primarylight sources.

FIGS. 5a-5f are sectional end views of an example cylindrical waveguideapparatus, with different combinations of photo-luminescent layer andreflector.

FIGS. 6a-6c are sectional side views of an example waveguide apparatus,showing various spacing between a scattering structure and itsassociated photo-luminescent layer.

FIG. 7 shows an application of the waveguide apparatus.

FIG. 8 shows another application of the waveguide apparatus.

DETAILED DESCRIPTION

This disclosure describes a waveguide apparatus suitable for anefficient distributed illumination system that can produce white lightat a location that is remote from a primary light source, and whoseradiation pattern (including its radial angle and radial spread) anduniformity is easily controllable along the waveguide length. Adiscussion of certain terms used here is first given, followed by adescription of various embodiments of the apparatus in relation to thefigures. Whenever the shapes, relative positions and other aspects ofthe parts described in the embodiments are not clearly defined, thescope of the invention is not limited only to the parts shown, which aremeant merely for the purpose of illustration. Also, while numerousdetails are set forth, it is understood that some embodiments of theinvention may be practiced without these details. In other instances,well-known structures and techniques have not been shown in detail so asnot to obscure the understanding of this description.

Wavelength—This term denotes the wavelength of the peak intensity of alight spectrum. For instance, this may relate to a quasi-singlewavelength or monochromatic source (e.g., a laser) or it may relate to abroader spectrum yet still narrow band light source (e.g., a singlecolor LED).

Primary light and primary light source—This refers to light that may beproduced by any radiation source that is capable of causing an emissionfrom a region of photo-luminescent material (also referred to here as aphoto-luminescent layer). For example, the primary source may be anincoherent, relatively broad spectrum, yet still “single color”, lightemitting diode, LED, that comprises an active region (p-n junction) thatmay include one or several quantum wells and may contain GaN, AlGaNand/or InGaN material. The primary source may alternatively be anorganic light emitting diode (OLED) or a source based on quantum dots.The primary source may alternatively be a coherent, sharp spectrum lightsource such as a laser emitting a single wavelength (also referred tohere as quasi single wavelength), or it may be multiple singlewavelength lasers, e.g. lasers emitting red, green and blue light (R, G,B), respectively.

Secondary light—This is light that is produced by a photoluminescenceprocess that responds to the primary light. In one instance, the primarylight is short wavelength (or “high” photon energy) light (e.g., green,blue or UV) that is absorbed by the photo-luminescent layer, while thesecondary light refers is long wavelength light (or “low” photon energy)that is re-emitted by the photo-luminescent layer. The secondary lightmay also be referred to here as wavelength converted light.

Illumination light—This term refers to light having at least a spectrumportion that is visible to the human eye and that is generated based ona photo-luminescence process (the secondary light) and may also includesome unabsorbed primary light. It may also have some components that arenot visible, e.g. infrared. The illumination light may have a spectralpower distribution similar to that of a white light emitting diode,WLED. The efficiency of the overall process for producing theillumination light may be enhanced when a wavelength of the primarylight matches with an absorption spectral band of the photo-luminescencelayer.

Color reproduction or color temperature—Color reproduction refers to ameasure of the quality of the colors produced by an illumination source,and that are visible to the human eye (photometry curve). An example isthe color rendering index, CRI. The color temperature is acharacteristic of the visible light as referred to the ideal of blackbody radiation. Color temperature is typically given in a chromaticitydiagram (CIE coordinates). In accordance with an embodiment of theinvention, the CRI of the illumination light may be adjusted to adesired one, by e.g., adapting the chemical composition of thephoto-luminescent medium. The color temperature may be adjusted byadapting the concentration (e.g., particle density) and/or thickness ofthe photo-luminescent medium. Other techniques for adapting the CRI, oralternatively the gamut, of the illumination light include doping thewaveguide, e.g. its core, with an active component such as those used inoptical fiber amplifiers and/or by using multiple, different colorprimary light sources.

Laser-induced scattering structures—This refers to the localmodification of the waveguide material by exposure to an external highenergy laser source. Such material modifications are not limited tolocal refractive index modifications but may also encompass true localmaterial modifications (melted structures or voids). External laserprocessing sources that may be used include deep UV lasers (CW orexcimer) that use the intrinsic photosensitivity of the waveguidematerial to locally modify the index of refraction; high peak powerfemtosecond lasers may be used in case the processed waveguide materialis not sufficiently photosensitive. For instance, periodic patterns maybe inscribed within a fiber core by exposing it to an intenseinterference pattern using an interferometer or a phase mask. Aperiodicscattering structures could be directly written inside the fiber coreusing the non-linear response of the glass matrix (preferred opticalfiber material) to intense laser light. The strength of a structure may,for example, be in the range Δn=10⁻⁷ to Δn=10⁻²(amplitude of change inindex of refraction). The strength may be higher where the structure hasmelted. The period of a scattering structure (e.g., grating period) maybe randomly selected, for example from the range 100 nm to 2 microns (inaccordance with the wavelength of the primary light).

Turning now to FIGS. 1a -1 b, these are sectional side views of awaveguide apparatus in accordance with an embodiment of the invention,showing various types of scattering structures formed therein. A primarylight 111, produced by a primary light source 116 (also referred to asan excitation light source, for reasons given below), in this example iscoupled into (e.g., an end surface of) and guided by a waveguide. Analternative here is to generate the primary light inside the waveguide,e.g. using a fiber laser structure that is formed along the waveguide(e.g., by exciting a doped region of a holey fiber filled with dye). Thewaveguide may be composed of a core 100 covered with a cladding 101. Thecore medium is in contact with the cladding medium, and these aredesigned such that the primary light (represented by λ_(p) in thefigures) can propagate in the core in the direction shown and along thelongitudinal axis of the waveguide. Here, the propagation is via totalinternal reflection. In one instance, the waveguide is an optical fiber,e.g. multi-mode, single-core single-clad fiber made of flexible glass,although other types of cylindrical waveguides are possible such asphotonic crystal fiber, micro-structure fiber, holey fiber, multi-cladfiber, and a light pipe having a core medium but no cladding layer, e.g.a transparent rod whose peripheral surface may be coated with a mirrorlayer except for a remote location where the primary light is to bere-directed outward, a transparent rod without a cladding layer andwhose peripheral surface is exposed to air such that the air acts as acladding for propagating the primary light. Note that in some types ofcylindrical waveguide, such as micro-structure fiber, the primary lightpropagates along the waveguide by a bandgap effect due to the periodicnature of the micro-structure fiber that forbids propagation indirections other than along the waveguide.

The waveguide has one or more scattering structures formed therein asshown, which serve to re-direct the propagating or incident primarylight out of a side surface of the waveguide. In other words, theprimary light is redirected to a desired non-zero angle (e.g.,transverse or about 90 degrees) relative to the longitudinal or opticalpropagation axis of the waveguide. The scattering structures may beconfinement regions that produce a resonance of the incident primarylight, in a traverse plane (resulting in a re-directed or scatteredprimary light that is coherent and that may exhibit a small wavelengthshift relative to the incident primary light). The scattering structuresmay be laser-induced structures; these may be formed through theapplication of external, high-energy laser light to selected locationsin the waveguide, as shown. The location, the shape, the size, thescattering strength, the tilt or orientation, and periodicity of thescattering structures, along and across (transverse to) the primarylight propagation direction (longitudinal axis) in the waveguide can beselected, by adapting the focus, intensity and position of the externalprocessing laser beam. The directionality of the re-directed primarylight (its radial angle about the longitudinal axis of the waveguide,and its radial spread) may be primarily a function of the tilt andperiod of the scattering structures, although additional parameters mayneed to be considered and balanced as a whole, to obtain the desiredre-directed primary light radiation pattern.

For instance, FIG. 1a shows a set of aperiodic scattering structures105, 106, 107. These may be micro-diffusers that are entirely inside thecore 100 (structures 105), in just a portion of the interface that joinsthe core 100 and the cladding 101 (structures 106), and/or crossing theentire core and cladding (structures 107). Any suitable combination ofsuch structures 105-107 may be used to define a scattering zone; forinstance, the scattering zone may be constituted entirely of just onetype of scattering structure, e.g. only inside-the-core structure 105.FIG. 1b shows a set of periodic scattering structures 108, 109, 110 suchas micro reflectors or tilted gratings. These may also be formedentirely inside the core 100 (structures 108), in just a portion of theinterface that joins the core 100 and the cladding 101 (structures 109),and/or crossing the entire interface (structures 110). Any suitablecombination of such structures 108-110 may be used to define ascattering zone; for instance, the scattering zone may be constitutedentirely of just one type of scattering structure, e.g. onlyentire-interface-crossing structure 110. In general, any suitablecombination and/or variation of one or more of the scattering structures105-110 that yields the desired radiation pattern for the re-directedprimary light, λ_(p), may be used. The length of a scattering zone(containing one or more scattering structures) may be the same as orsmaller than the length of the photo-luminescent layer 103 orphoto-luminescent layer segment that is associated with the zone.Alternatively, the scattering zone may extend further than thephoto-luminescent layer 103 or the photo-luminescent layer segment.

Still referring to FIGS. 1a -1 b, the waveguide apparatus has aphoto-luminescent layer (medium) 103 which is located so as topartially, or completely, absorb the re-directed primary light λ_(p) tothereby emit secondary light 112 (λ_(s)) in accordance with aphoto-luminescence process. This is also referred to as wavelengthconverted light. The resulting combination of this secondary light 112and any unabsorbed primary light (λ_(p); λ_(s)) is the desiredillumination light 113. Such an illumination system may provide specialadvantages in the area of cylindrical waveguide-based distributedillumination systems, including improved energy efficiency, scalability,and homogeneous light output.

The photo-luminescent layer 103 may be made of a mixture of silicone(e.g., as part of an optically clear adhesive such aspolydimethylsiloxane, PDMS) or other suitable material such as epoxy,together with a phosphor. The concentration of the phosphor and the sizeof the phosphor particles may be selected to modify or control the colortemperature and luminous efficiency of the illumination system. Note the“phosphor” as used here refers to any material that exhibitsluminescence, e.g. phosphorescent materials, fluorescent materials. Thelayer 103 may be composed of one or more layers of differentcompositions. For instance, there may be one or more intermediatenon-phosphor layers sandwiched by phosphor layers.

A protection layer 114 may be added, in this case in contact with theouter side surface of the photo-luminescent layer 103, to cover thelatter for purposes of physical protection and/or reduction of the indexof refraction step between the layer 103 and the outer medium, i.e.external to the waveguide apparatus. See FIG. 3a . The protection layer114 may also contain or simply be an anti-reflection coating, serving toreduce or minimize any back-reflection of the secondary light λ_(s) offits back or rear surface.

FIG. 1c is a graph of an example spectrum of the illumination light 113provided by the waveguide apparatus. As seen here, the illuminationlight 113 has the spectrum I(λ_(p); λ_(s)), where the bandwidth, e.g.full width at half maximum (FWHM), of secondary light is given asΔλ_(s), and that of the primary light is Δλ. In most instances, Δλ_(s)will be about the same or larger than Δλ_(p), λ_(p) will be shorter thanλ_(s), i.e. has higher photon energy, and λ_(s) will predominantly be inthe visible part of the spectrum. In this example, the primary light isfrom a single wavelength source and is centered at between 440-490 nm,namely blue, with a bandwidth of between a few tenths of picometers(quasi-single wavelength, as may be obtained by a laser diode), to a fewtens of nanometers (LEDs, or even a super-luminescence diode which canbe wider than 100 nm). Other desirable options for the wavelength of theprimary light include ultraviolet (300-400 nm), and violet (400-440 nm).More generally, however, the bandwidth and wavelength parameters couldbe different than the above, e.g. the primary light may be green, red,or even partially in the non-visible portion of the spectrum, e.g. nearinfrared.

As to the secondary light, FIG. 1c shows λ_(s) centered at about 550 nm.This is typical of emissions by many popular types of phosphor layermaterial, e.g. Ce:YAG. For white light, a desired bandwidth of theillumination light 113 is between 380-740 nm. There may be someinvisible content generated by the photoluminescence process and so inpractice the bandwidth of the illumination light could be larger than380-740 nm.

The photo-luminescent layer 103 may be located outside the waveguide asshown in FIG. 1 a. The layer 103 may be deposited directly on theexternal side surface of the waveguide, to form an integral or singledevice with the waveguide—see FIG. 3a for an example.

Alternatively, an intermediate layer 102 may be provided between thephoto-luminescent layer 103 and waveguide; it may serve to adapt anotherwise step in refractive index difference, between the waveguide andthe photo-luminescent layer 103, to enable more efficient outcoupling ofthe redirected primary light (less reflection), and may be made of oneor more sub-layers of glass, silicone, other suitably optically clearmaterial. It may also include or be an anti-reflection coating thathelps enhance the light transmission efficiency of the illuminationsystem as a whole, by re-directing any back-reflected secondary lightλ_(s), i.e. secondary light that has been reflected by the back facephoto-luminescent layer 103. The layer 102 may also be used to join thephoto-luminescent layer 103, which may be a separate optical piece, tothe waveguide, e.g. as an optically clear adhesive layer. Theintermediate layer 102 may have a thickness similar to that of thephoto-luminescent layer 103, but in some cases may be much thinner suchas in the case where it is merely an anti-reflection coating or filter.

In another embodiment, an air gap is formed between the outer sidesurface of the waveguide (here, the outer side surface of the claddinglayer 101) and the photo-luminescent layer 103, through which there-directed primary light λ_(p) passes before striking thephoto-luminescent layer 103.

In order to lower the parasitic reflection at the interface between thewaveguide and the photo-luminescent layer 103, the shape of thephoto-luminescent layer 103 may conform to the radiation pattern of there-directed primary light such that the primary light is incident uponthe layer 103 transversely. In one embodiment, the layer 103 conforms tothe waveguide such that it follows or conforms, or has the same shapeas, the external side surface of the waveguide. For instance, in thecase of an optical fiber, the layer 103 could have a cylindrical shapeand may also be positioned to be concentric to the optical fiber—seeFIG. 3a . Alternatives are possible, for example as described below inconnection with FIG. 5b and FIG. 5d . In general, the photo-luminescentlayer 103 need not entirely cover the region along the longitudinal axisof the waveguide that scatters the primary light, i.e. some of thescattered primary light could bypass the photo-luminescent layer 103 andstill contribute to the desired illumination. The length of thephoto-luminescent layer 103 or photo-luminescent layer segment may beequal to or larger than its associated scattering zone, and may alsodepend on the angle of the incident, re-directed primary light.

The primary light strikes the photo-luminescent layer 103 at an anglethat may be defined or fixed by the shape of the scattering structurethat re-directed it; this may be designed to achieve the desiredconversion by the photoluminescence process. In particular, thescattering efficiency, the distribution and the directionality of theradiated primary light are given by a combination of certaincharacteristics of the guided primary light (e.g., its wavelength, stateof polarization, modal distribution) and certain parameters of thescattering structures (e.g., their magnitude, shape, and periodicityalong/across the propagation axis). FIGS. 2a-2c are sectional views ofexample radiation patterns for the re-directed primary light, showingthree possible combinations of fiber and scattering structure parametersthat provide three different scattering distributions for the primaryhigh energy light. The radiation pattern runs along the longitudinalaxis (z-direction), according to the length of the scattering zone.

In FIG. 2a , a single micro-diffuser has been processed inside the core100 of the waveguide. The primary light 111 is launched into the core100 in, for example, a linear state of polarization at zero degrees, inthe direction of the positive z-axis (into the paper), and the resultingscattered light distribution is shown. In this case, there are tworadiation lobes opposing each other, each forming a large scatteringcone having a radial spread a. In contrast, FIG. 2b shows a pattern thatcan be produced by, e.g. a tilted grating formed inside the core 100.Here, the primary light wavelength is chosen to be at an edge of thescattering wavelength band of the tilted grating (partially coherentscattering). The scattered light distribution here also presents twomain lobes together forming a medium size cone of scattering angle α.Finally, in FIG. 2c , the tilted grating has been designed such that theprimary light wavelength is at a location of the scattering wavelengthband of the tilted grating where there is maximum scattering (coherentscattering). The resulting scattered light distribution has a singlemain lobe whose angle α is smaller than in FIG. 2a . For manyapplications, the scattering structures should be designed to yield aradiation pattern, for the re-directed primary light, that consists of asingle lobe (similar to FIG. 2c ) of up to 180 degrees of radialcoverage or spread.

Note that generally speaking, a scattering structure is dispersive inthat different frequency or color bands will be scattered at differentangles. However, in most instances, the scattering bandwidth will bemuch broader than the relatively narrow bandwidth of the primary lightcontemplated here.

FIG. 3a is a sectional end view (in the azimuthal plane) of an examplecylindrical waveguide apparatus. FIG. 3b is a sectional end view of theexample waveguide apparatus of FIG. 3a superimposed with references tothe behavior of the primary and secondary light. This is also an exampleof what a “fully-integrated” version of the waveguide apparatus mightlook like, where the elements of the apparatus have been combined withthe waveguide itself to form in essence a single device. The waveguideapparatus includes a set of laser-induced scattering structures that maybe any combination of the scattering structures 105-110 described above,formed in this case entirely inside the core 100 of the optical fiber.The core 100 is surrounded by a cladding 101, that enables the primarylight to propagate along the fiber. The photo-luminescent layer 103 inthis case may have been formed, e.g. deposited, sputtered, or evaporatedto be in contact with the external surface of the cladding 101, andextends to cover at least the azimuthal scattering extent a of theprimary light, as shown. As an alternative, the fiber may be embeddedinto the photo-luminescent medium—see FIG. 5c described below.

In this example, a reflector 104 has also been formed, e.g. deposited,sputtered, or evaporated in contact with the intermediate layer 102,positioned behind the photo-luminescent layer 103, and also in this casebehind the fiber itself. Here, the reflector 104 is a layer that isconcentric with the fiber. The reflector 104 may be a layer of a highlyreflective polymer, e.g. polyphthalamide, or a layer of aluminum. It maybe a dielectric layer coating on the fiber, e.g. may be deposited ontothe intermediate layer 102. Alternatively, the reflector 104 may bepositioned apart from the fiber, as part of a separate piece.

As seen in FIG. 3b , the primary light is partially scattered out of thefiber core 100 by the scattering structures 105-110, over a definedangle α. The primary light is then absorbed by the photo-luminescentlayer 103, and re-emitted over a broader light spectrum. The primarylight that is not directly absorbed by the photo-luminescent layer thenilluminates an area outside the fiber (1). The secondary light may alsodirectly illuminate outside the fiber (2), or it may do so indirectly,by reflection from the bottom reflector (3), or by partial guiding (4)within the photo-luminescent layer 103 and then off the reflector 104. Arole of the intermediate layer 102 may be to adapt the refractive indexdifference between the fiber cladding material and the photo-luminescentlayer 103. A role of the protection layer 114 is to facilitate theintegration of the waveguide apparatus in the illumination system byreducing the index of refraction difference between thephoto-luminescent layer 103 and the refractive index of the external tothe waveguide apparatus. The protection layer 114 and the intermediatelayer 102 may or may not be needed, as part of the waveguide apparatus,depending on the integration demands.

Turning now to FIG. 4, this is a perspective view of an examplecylindrical waveguide apparatus for being coupled to multiple, differentwavelength primary light sources. In this embodiment of the invention,the apparatus has a photo-luminescent layer 103 whose longitudinalextension is non-continuous or non-uniform, and is composed of threesegments 103 a, 103 b, and 103 c. Also, multiple (different) wavelengthprimary light sources, in this example, three different colorsingle-wavelength sources λ_(p1), λ_(p2), λ_(p3), are coupled into andguided by the core 100 of the cylindrical waveguide. A separate set ofone or more scattering structures 105-110 is designed to respond to eachrespective primary light wavelength, or to a set of two or more primarylight wavelengths. Also, each of the photo-luminescent segments 103a-103 c may be designed to respond to a respective primary lightwavelength, or to a set of two or more primary light wavelengths. Notethat in general, this concept can be extended to two or more primarylight sources at different wavelengths.

In FIG. 4, a separate set of scattering structures 105-110 is providedto scatter out primary light of wavelength λ_(p1) towards segment 103 a,such that a broad wavelength band of illumination light 113 a,containing λ_(p1) and the associated secondary light λ_(s1), is emittedas a result. Similarly, a separate set of scattering structures 105-110(not shown) scatters out primary light of wavelength λ_(p2) towardssegment 103 b, while a yet further set of scattering structures 105-110(not shown) scatters out primary light of wavelength λ_(p3) towardssegment 103 c. Thus, three, broad wavelength bands of illumination light113 a, 113 b, 113 c are emitted as a result. This embodiment may improvethe CRI and/or extend the overall spectrum of the illumination light,where λ_(p1), λ_(p2), λ_(p3) are produced by red, green, and blueprimary light sources, respectively. It should be noted that in somecases, one or more of the photo-luminescent layer segments 103 a-103 cmay be selected to be absent, such that in that region of the waveguideapparatus the re-directed primary light directly illuminates a desiredregion outside the waveguide apparatus, without any interaction with aphoto-luminescent medium. For example, one phosphor-covered segment ofthe waveguide apparatus may produce white light, while a “clear” segment(spaced longitudinally along the waveguide) is producing blue light(because the re-directed primary light is not interacting with aphosphor medium in that segment). This may be used to tune the CRI orcolor temperature of the illumination light, by mixing together outsidethe waveguide apparatus both the secondary light and any desired primarylight. The color temperature may also be tuned by adjusting theintensity of any one of the primary light sources relative to theothers.

It should be noted that while FIG. 4 depicts a multi-segmentphoto-luminescent layer 103 a-103 c that is excited by primary lighthaving multiple discrete wavelengths λ_(p1), λ_(p2), λ_(p3), thismulti-segment approach may also be useful for tuning the resultingillumination light (λ_(p), λ_(s)) in the case where the primary light isjust single-wavelength.

The overall conversion efficiency of the illumination light generationprocess described here may depend on several factors, including theelectro-optical efficacy of the primary light source, the Stokesconversion efficiency, the quantum efficiency of the photo-luminescencelayer, and the “package” efficiency. The package includes the geometryof the waveguide apparatus as a whole, and the distinct materialinterfaces involved along the path traveled by the light, from its “highenergy state” upon being launched by the primary light source, to its“wavelength converted state” upon being re-emitted by thephoto-luminescent layer.

Referring now to FIGS. 5a -5 f, these are sectional end views of anexample cylindrical waveguide apparatus, with different combinations ofphoto-luminescent layer 103 and reflector 104. In all of these figures,the reflector 104 is said to be behind the waveguide (composed of core100 and cladding 101), while the photo-luminescent layer 103 is said tobe in front of the waveguide. In FIG. 5a and FIG. 5b , the reflector 104has a smooth inner curved surface facing a rear of the waveguide and hasa larger radius than the waveguide. In FIG. 5a , an inner surface of thephoto-luminescent layer 103 is entirely in contact with the outersurface of the waveguide, while its outer surface also generallyconforms to the outer surface of the waveguide. Here, the layer 103 mayhave a thickness of a few microns to a few millimeters. In contrast, inFIG. 5b , the inner surface of the layer 103 is not in contact with thewaveguide outer surface, and its outer surface does not conform to theouter surface of the waveguide. In FIG. 5c , the waveguide is embeddedin what appears to be a pool of the photo-luminescent layer/medium 103carried by the bottom and side walls of the reflector 104, whereessentially the entirety of the outer surface of the waveguide is incontact with the layer 103. The reflector 104 in this case may be viewedas a vessel in which the layer/medium 103 is contained, because theinner surface of the reflector 104 is in contact with the outer surfaceof the layer 103 all around the bottom of the layer 103. In FIGS. 5d -5f, the reflector 104 also defines a vessel that is carrying thewaveguide, and there may be an air gap between the waveguide and thephoto-luminescent layer 103. In FIG. 5d , the layer 103 is attached toand conforms to the shape of a protection layer 114 located in front ofthe layer 103; together these two form an arch that may be viewed asresting on the side walls of the reflector 104. In FIG. 5e and in FIG.5f , the layer 103 is flat rather than curved, and closes off the top ofthe trench-like reflector 104; in FIG. 5, the layer 103 is formed on theback surface of a light guide sheet 115.

Other combinations of the waveguide, photo-luminescent layer/medium 103and reflector 104 are possible. For instance, the photo-luminescentmedium 103 may be a curved layer, e.g. cylindrical layer, formed on theoutside surface of a cylindrical optical piece, where the lattersurrounds a rectangular waveguide (with a scattering structure such as atilted grating formed inside the rectangular waveguide).

FIGS. 6a-6c are sectional side views of an example waveguide apparatus,showing various spacing that may be possible between a scatteringstructure 105-110 and its associated photo-luminescent layer 103. Asabove, the primary light source 116 produces primary light 111 that iscoupled into the core 100 of a cylindrical waveguide having a cladding101. The primary light propagates along and within the waveguide untilit is redirected by the scattering structure 105-110. In FIG. 6a , thisredirection is about 90 degrees relative to the longitudinal orpropagation axis of the waveguide, or the re-directed primary light istransverse to the longitudinal or propagation axis. Also, thephoto-luminescent layer 103 is positioned directly above the scatteringstructure, so as to present an active surface that is preferablyoriented at about 90 degrees relative to the re-directed light. Incontrast, in FIG. 6b , the scattering structure is such that there-directed primary light is scattered forward, and into the cladding101 (rather than transverse to the propagation axis), and is alsolongitudinally spaced rearward from the layer 103 (rather than beingdirectly underneath the layer 103). To achieve such forward scattering,the periodicity of the scattering structures may be increased to above 1micron and in accordance with the wavelength of the primary light. Inanother variation of such “offset” between the scattering zone and itsassociated photo-luminescent segment (layer 103), shown in FIG. 6c , there-directed primary light is scattered rearward and into the cladding101, and the scattering structure is longitudinally spaced forward fromthe layer 103. The latter two arrangements may be less efficient inproducing the illumination light 113 than the arrangement in FIG. 6a .However, they may be needed to meet specific integration or packagingrequirements, albeit at a cost of lower power efficiency in producingthe illumination light. The longitudinal separation between the adjacentedges of a scattering structure and its associated layer 103 may be inthe range of a few microns or more.

Turning now to FIG. 7, a diagram of a backlighting application of thewaveguide apparatus is shown. The backlighting system in this examplehas a stackup as follows: a display element array 305 (e.g., a matrix ofcolor filters as in a liquid crystal display, LCD, panel) whose rearfaces one or more prism sheets 301, followed by a diffuser sheet 302,and a light guide sheet 303, e.g. a backplate. Behind the light guidesheet 303 is a reflector sheet 304, which serves to reflect back anyillumination light (λ_(p); λ_(s)) towards the display element array 305.The illumination light (λ_(p); λ_(s)) may be produced as a cone, e.g. ofless than 180 degrees, by a cylindrical waveguide apparatus (referencedhere as the combination of elements 100-110 described above inaccordance with one of the embodiments of the invention), that radiatesout of the outer side surface of the waveguide and is injected into aside surface of the light guide sheet 303 as shown. The light guidesheet 303 then directs the injected illumination light upward andspreads it onto and across the back face of the display element array305. Note that other backlighting systems are possible, including onesthat do not have a display element array, e.g. a plain, edge-lightingsystem.

FIG. 8 shows another display system in which the backlightingillumination light (λ_(p); λ_(s)) is produced within the stackup of abacklighting system, rather than being produced outside (by a waveguideapparatus) and then injected into a side surface of the stackup. Thewaveguide apparatus in this case may be an optical fiber similar to theone depicted in FIG. 3a , except that the photo-luminescent medium 103is absent. The waveguide apparatus in this case may contain only thefollowing cylindrical waveguide elements: core 100, cladding 101,scattering zone containing a combination of one or more structuresselected from the group consisting of structures 105-110, and reflector104 (with the intermediate layer 102 and protection layer 114 beingoptional). The re-directed primary light λ_(p) (which may also have anelongated cone shaped radiation pattern of less than 180 degrees,similar to those depicted in FIGS. 2a-2c ) emerges from aside-scattering elongated waveguide and is then injected into the outerside surface of a special light guide sheet 306. The latter has an outerupper face that has been coated with a layer of photo-luminescentmaterial (shown in FIG. 8 as shading). The special light guide sheet 306then directs the injected primary light upward and spreads it onto andacross the back face of the layer of photo-luminescent material, whichin turn emits secondary light λ_(s). The latter together with anyunabsorbed λ_(p) provide the desired illumination light that is incidenton the back face of the display element array 305.

A process for manufacturing the waveguide apparatus may be as follows.Unless specifically required, the operations here do not have to be anyparticular order. A region of light scattering structures is formedwithin a waveguide extending along a longitudinal axis of the waveguide.The region of scattering structures is adapted to re-direct a primarylight, that will be propagating in the waveguide along the longitudinalaxis, out of an outer side surface of the waveguide. A medium ofphoto-luminescent material is formed that is positioned outside thewaveguide to absorb the re-directed primary light and thereby emitsecondary light.

The forming of the region of light scattering structures may includelaser processing the waveguide, to create the region of light scatteringstructures therein. The laser processing may take place before formingthe photo-luminescent medium. The laser processing may include changingfocus intensity and position of a processing laser beam, that is aimedat the waveguide, to set one or more of the following parameters of thelight scattering structures: location, shape, size, orientation or tilt,and periodicity. The laser processing may thus be adapted to yield thedesired scattering structures, having a particular scattering strengthand directionality. For example, the scattering structures within agiven region may be written so that the scattering strength of theregion is less at a proximal point than at a distal point along thelongitudinal axis of the waveguide; this may help improve the uniformityof the intensity of the white light along the waveguide, by compensatingfor the inevitable losses in the primary light as it propagates throughthe scattering region.

The medium of photo-luminescent material may conform to the outer sidesurface of the waveguide. For example, formation of the medium ofphoto-luminescent material may include creating a layer of thephoto-luminescent material on the side surface of the waveguide. Anintermediate layer may be formed on the side surface of the waveguide,prior to the layer of the photo-luminescent material being formed (onthe outer side surface of the intermediate layer). The process may use amixture of silicone-phosphor to form the photo-luminescent layer on anoptical fiber; the mixture can be made in a separate operation and thendispensed (or thermo-dispensed, sputtered, or evaporated) onto thepreviously manufactured fiber. As an alternative, the silicone-phosphormixture can be made directly on the outer side surface of the fiber. Forinstance, the silicone may be dispensed onto the fiber and then thephosphor can be sputtered on it. A thermal annealing operation mightthen be necessary for the polymerization. A membrane or film of themixture can be made in a separate operation and then deposited or bondedonto the fiber for better control of the layer thickness.

An alternative is to at least partly (e.g., entirely) embed thewaveguide into a pool of the photo-luminescent medium.

The intermediate layer formed between the waveguide and the medium ofphoto-luminescent material may be adapted to increase efficiency ofoutcoupling of the re-directed primary light, e.g. by having multiplelayers of light passing material with index of refractions chosen toadapt the otherwise step in refractive index between thephoto-luminescent medium and the waveguide.

The process may also include the formation of a reflector behind thewaveguide and that faces the medium of photo-luminescent material (whichis considered in that case to be in front of the waveguide). Thereflector may be V-shaped or curved, e.g. U-shaped. The reflector mayinclude a reflective layer that conforms to the outer side surface ofthe waveguide and is designed to reflect both the primary light and thesecondary towards the photo-luminescent medium.

While certain embodiments have been described and shown in theaccompanying drawings, it is to be understood that such embodiments aremerely illustrative of and not restrictive of the broad invention, andthat the invention is not limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those of ordinary skill in the art. For example, although thefigures show the photo-luminescent layer 103 being outside of thecladding 101 of the waveguide, and hence not in contact with the core100, a less desirable alternative is to chemically or mechanicallyremove some portion of the cladding 101 to create in effect a trenchthat exposes the core 100, and then fill the trench withphoto-luminescent material. The description is thus to be regarded asillustrative instead of limiting.

What is claimed is:
 1. A waveguide apparatus for an illumination system, comprising: a waveguide in which a core medium is in contact with a cladding medium and in which primary light is to propagate in the core medium along its longitudinal axis under total internal reflection against the cladding, the waveguide having formed therein a first zone of scattering structures, being a laser-induced modification of at least the core medium that runs along the longitudinal axis and that is to scatter the propagating primary light out of the waveguide in a directional radiation pattern having at least one lobe whose predetermined radial spread of less than 360 degrees is at a desired radial position, wherein scattering strength of the first zone varies as a function of position along the longitudinal axis of the waveguide; and a photo-luminescent medium that runs along the longitudinal axis of the waveguide, and is positioned to absorb the scattered primary light to thereby emit a secondary wavelength converted light.
 2. The apparatus of claim 1 wherein the photo-luminescent medium conforms to or has the same shape as a side surface of the waveguide and extends around the entire circumference of the waveguide.
 3. The apparatus of claim 2 further comprising an elongated reflector running along the longitudinal axis, that is outside of the waveguide_and facing the photo-luminescent medium.
 4. The apparatus of claim 3 further comprising: a light source to produce the primary light, the light source being coupled to an end surface of the waveguide to couple the primary light into the waveguide, wherein the primary light is to propagate within the waveguide, in a downstream direction along the longitudinal axis of the waveguide, until it is redirected by the first zone at a selected location that is downstream of where the light source is located.
 5. The apparatus of claim 2 further comprising: a light source to produce the primary light, the light source being coupled to an end surface of the waveguide to couple the primary light into the waveguide, wherein the primary light is to propagate within the waveguide, in a downstream direction along the longitudinal axis of the waveguide, until it is redirected by the first zone at a selected location that is downstream of where the light source is located.
 6. The apparatus of claim 1 further comprising: a light source to produce the primary light, the light source being coupled to an end surface of the waveguide to couple the primary light into the waveguide, wherein the primary light is to propagate within the waveguide, in a downstream direction along the longitudinal axis of the waveguide, until it is redirected by the first zone at a selected location that is downstream of where the light source is located.
 7. The apparatus of claim 6 wherein the primary light produced by the light source is in a non-visible spectrum.
 8. The apparatus of claim 1 further comprising an intermediate medium between an outer side surface of the waveguide and the photo-luminescent medium.
 9. The apparatus of claim 8 wherein the intermediate medium is to adapt the step in refractive index difference between the waveguide and the photo-luminescent medium, so as to improve the efficiency with which the primary light is extracted from the waveguide and strikes the photo-luminescent medium and to improve the extraction efficiency of the secondary light out of the photo-luminescent medium.
 10. The apparatus of claim 1 wherein an air gap is formed between an outer side surface of the waveguide and the photo-luminescent medium.
 11. The apparatus of claim 1 wherein the scattering strength of the first zone is defined by a change in index of refraction Δn in the range Δn−10⁻⁷ to Δn=10⁻².
 12. The apparatus of claim 1 wherein the scattering strength of the first zone varies as a function of position along the longitudinal axis of the waveguide in a way that compensates for power loss that would be suffered by the primary light as it propagates along the first zone.
 13. The apparatus of claim 1 further comprising an elongated reflector running along the longitudinal axis, that is outside and facing an outer side surface of the waveguide.
 14. The apparatus of claim 1 further comprising a second scattering zone that a) is positioned further downstream than the first zone and b) has greater scattering strength than the first zone when re-directing the primary light, wherein the photo-luminescent medium extends further along the longitudinal axis of the waveguide and is positioned to absorb scattered primary light from the second zone to thereby emit further secondary wavelength converted light.
 15. A method for manufacturing a waveguide apparatus for an illumination system, comprising: forming a region of light scattering structures within a waveguide in which a core medium is in contact with a cladding medium and in which primary light is to propagate along its longitudinal axis, by laser processing at least the core medium along the longitudinal axis to create therein the region of light scattering structures wherein the region of light scattering structures is adapted to re-direct a primary light, that is propagating in the waveguide along the longitudinal axis, out of an outer side surface of the waveguide in a directional radiation pattern; and creating a layer of photo-luminescent material on an entirety of an outer side surface of the waveguide, wherein the layer of photo-luminescent material is to absorb the re-directed primary light and thereby emit secondary light.
 16. The method of claim 15 wherein a scattering strength of the region of light scattering structures is given by a change in index of refraction Δn in the range Δn=10⁻⁷ to Δn=10⁻².
 17. The method of claim 16 wherein said laser processing comprises using a laser interferometry technique to write periodic scattering structures in the waveguide.
 18. The method of claim 15 wherein the laser processing comprises: changing focus intensity or position of a processing laser beam that is aimed at said waveguide to set one or more of the group consisting of location, shape, size, orientation or tilt, and periodicity of the light scattering structures, so that a scattering strength of the region of light scattering structures is less at a proximal point than at a distal point along the longitudinal axis of the waveguide in a way that compensates for power loss that would be suffered by the primary light as it propagates along said region.
 19. The method of claim 18 further comprising: forming a reflector behind the waveguide.
 20. A light waveguide apparatus comprising: waveguide means in which a core medium is in contact with a cladding medium for guiding primary light that propagates in the core medium in a longitudinal direction under total internal reflection against the cladding medium, and having formed therein means for re-directing the primary light out of the waveguide means, the re-directing means being a zone of scattering structures that is a laser-induced modification of at least the core medium that runs along the longitudinal direction and that is to scatter the primary light out of the waveguide means in a directional radiation pattern having at least one lobe, wherein scattering strength of the zone varies as a function of position along the longitudinal axis of the waveguide in a way that compensates for power loss that would be suffered by the primary light as it propagates along said zone; and photo-luminescent means for absorbing the re-directed primary light to thereby emit secondary wavelength converted light having a different wavelength than said primary light.
 21. The light waveguide apparatus of claim 20 wherein the photo-luminescent means is a non-uniform layer having a plurality of different composition segments that are arranged side by side along the longitudinal direction of the waveguide means.
 22. The light waveguide apparatus of claim 20 wherein the photo-luminescent means is a non-uniform layer having a plurality of different thickness segments that are arranged side by side along the longitudinal direction of the waveguide means.
 23. The light waveguide apparatus of claim 20 wherein the photo-luminescent means is a non-uniform layer having a plurality of segments which together configure the light waveguide apparatus to have at least one phosphor-covered segment and at least one clear segment.
 24. The light waveguide apparatus of claim 20 further comprising reflector means located behind and facing the waveguide means.
 25. The light waveguide apparatus of claim 20 further comprising: a light guide sheet means positioned relative to the waveguide means so that white light, consisting essentially of any unabsorbed re-directed primary light combined with the secondary wavelength converted light outside of the waveguide means, is coupled into a side surface of the light guide sheet means.
 26. The light waveguide apparatus of claim 20 further comprising a light guide sheet means wherein the photo-luminescent means is a layer formed on a front face of the light guide sheet means, and wherein the light guide sheet means is positioned relative to the waveguide means so that the re-directed primary light is coupled into a side surface of the light guide sheet means and is then directed to the front face, and wherein white light consisting essentially of any unabsorbed re-directed primary light combined with the secondary wavelength converted light emerges from the front face of the light guide sheet means.
 27. The light waveguide apparatus of claim 20 wherein the photo-luminescent means is a photo-luminescent layer having at least one absent segment such that in that region of the light waveguide apparatus the re-directed primary light directly illuminates a desired region outside the light waveguide apparatus without interaction with a photo-luminescent medium.
 28. The light waveguide apparatus of claim 20 further comprising: a light source to produce the primary light, the light source being coupled to an end surface of the waveguide means to couple the primary light into the waveguide means, wherein the primary light is to propagate within the waveguide means in a downstream direction along the longitudinal direction until it is redirected by the laser-induced modification of the core medium at a selected location that is downstream of where the light source is located.
 29. The light waveguide apparatus of claim 20 wherein the scattering strength of the zone of scattering structures is given by a change in index of refraction Δn in the range Δn=10⁻⁷ to Δn=10⁻². 